Implication of increased live loads on the design of precast concrete bridge girders

Abstract

American Association of State Highway and Trans- portation Officials (AASHTO) design live loads are sometimes increased as directed by relevant authorities to reflect actual traffic conditions. A 50% increase is adopted in some regions of the Middle East based on comparisons of AASHTO LRFD specifications HL93 live loads with the British Standards HA + HB live loads. Even in some U.S. jurisdictions, the AASHTO live loads may not represent modern truck configura- tions. This paper examines the effects of increasing AASHTO LRFD specifications HL93 live loads on the design of precast, prestressed concrete girders. A parametric study was first conducted for this purpose. Design aids in the form of charts that relate the precast concrete girder size and spacing to the span length as a function of the 28-day concrete compressive strength and environmental classifications were developed for HL93 and 1.5 × HL93 live loads. It was shown that the increase in live loads can be economically accom- modated by increasing the 28-day concrete strength. The charts not only set up the basis of comparisons but also provided a practical solution that can be simply used by precast concrete designers for optimizing the precast concrete girder size and spacing.